Coloring the Universe: How Beautiful Astronomical Images are Made
Figure 1: The cover of Coloring the Universe, showing an optical image from the NSF’s Mayall 4-meter telescope at Kitt Peak National Observatory of IC 1396A, a dark nebula more commonly known as the Elephant Trunk Nebula. Credit: T.A. Rector (University of Alaska Anchorage) and H. Schweiker (WIYN and NOAO/AURA/NSF).
The text in Coloring the Universe is eloquent and accessible to a wide audience. The book has excellent organization, and each chapter is broken up into easily digestible subsections. Some of the topics covered include a comparison of human vision with telescopic vision and a discussion of what astrophysics can be learned from images. It also explains some details about observing at the world’s largest telescopes and discusses the different kinds of light that we observe.
|Figure 2: An HST image showing part of the Cygnus Loop supernova remnant, the expanding remains of a massive star that exploded about 8,000 years ago. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) |
|Figure 3: An optical image from the Mayall 4-meter telescope, of the region known as Pickering's Triangle, also part of the Cygnus Loop supernova remnant. This image is rotated by 180 degrees from the one used in the book. Credit: T.A. Rector (University of Alaska Anchorage) and H. Schweiker (WIYN and NOAO/AURA/NSF).|
In this case and many others, the much bigger field of view of ground-based telescopes compared to HST can compensate for their much lower spatial resolution, as I described in a blog post in 2014: “What Makes an Astronomical Image Beautiful?”, based on a paper by Lars Lindberg Christensen and colleagues. One reason that HST images are often more familiar is that they have a more powerful publicity engine promoting them. Coloring the Universe allows part of this publicity imbalance to be rectified.
Figure 4: An optical image using the NSF's 0.9-meter telescope at Kitt Peak National Observatory of the Rosette Nebula. Credit: T.A. Rector, B.A. Wolpa & M. Hanna (NOAO/AURA/NSF).
Figure 5: A composite image of NGC 602, a cluster of bright young stars in the Small Magellanic Cloud, a nearby galaxy. Chandra data is shown in purple, optical data from HST is shown in red, green and blue and infrared data from the Spitzer Space Telescope is shown in red. Credit: X-ray: NASA/CXC/Univ.Potsdam/L.Oskinova et al; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech
As various image experts have explained over the years, such as Megan Watzke, Kim Arcand and Robert Hurt, the colors used in astronomical images are often representative, which means that they are not intended to show how our eyes might see the object, but instead represent maps of the electromagnetic radiation produced at different wavelengths, and with a range of filters. Astronomers often use the term “false color”, but that terminology is misleading for non-expert audiences, as Robert Hurt and others have pointed out. In many cases images simulate what our eyes might see if they were sensitive to very different wavelengths, like Geordi La Forge's visor-enhanced vision in Star Trek the Next Generation.
Figure 6: A Chandra image of Tycho’s supernova remnant. Credit: NASA/CXC/SAO
- It shows a sense of humor, e.g. in describing how colors should be used appropriately it notes that people in images wouldn’t be colored green “unless you’re in Roswell, New Mexico”. Also, since optical astronomers observe (hopefully) all night, the authors note, “like vampires we sleep during the day” and “Fortunately, we don’t sleep in coffins”.
- I liked the use of raw images to show what telescopes collect before processing has been done, and how calibration and multiple exposures correct for changes in charge coupled detector (CCD) sensitivity and gaps between CCDs.
- It gives an excellent description of the history of astronomical images and their dissemination, including HST’s observations of Jupiter and Shoemaker-Levy in 1994 at a pivotal time for the production and dissemination of images. The World Wide Web started in 1991 but its use was limited until the Mosaic web browser was introduced in 1993. Another big step was the release of layering capabilities in Photoshop in 1994, allowing sophisticated color composite images to be created.
- An authoritative description is provided of the important role that images play in publicity, including the establishment of programs like Hubble Heritage, providing the opportunity to gather beautiful images that professional astronomers might have missed.